Broadband optical network apparatus and method

ABSTRACT

Methods and apparatus for providing enhanced optical networking service and performance which are particularly advantageous in terms of low cost and use of existing infrastructure, access control techniques, and components. In the exemplary embodiment, current widespread deployment and associated low cost of Ethernet-based systems are leveraged through use of an Ethernet CSMA/CD MAC in the optical domain on a passive optical network (PON) system. Additionally, local networking services are optionally provided to the network units on the PON since each local receiver can receive signals from all other users. An improved symmetric coupler arrangement provides the foregoing functionality at low cost. The improved system architecture also allows for fiber failure protection which is readily implemented at low cost and with minimal modification.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates generally to an optical network system, includingfor example a broadband passive optical network (B-PON). In particular,the invention describes an optical network system that allowsalternative access techniques (such as for example an Ethernet-typeCSMA/CD MAC) to be implemented optically on the network.

2. Description of Related Technology

While optical networking as an industry has experienced significantgrowth over the past few years, this growth is mainly focused on longhaul or backbone optical networks. Although new technologies that allowcost-effective, scalable, reliable, high-bandwidth services are emergingin the metro and regional market, little has changed in the accessarena. The ever-increasing demand for bandwidth has accelerated the lagof subscriber access network capacity. In other words, the bandwidthbottleneck has evolved outward from the core to the subscriber accessnetwork, or the so-called “first mile.” In order to be able to providethe new services that customers demand, service providers need to findways to offer higher data rates at reasonable costs.

Several technologies exist today that are being used to increase thecapacity of subscriber access networks. For example, hybridfiber-coaxial (HFC) and digital subscriber line (DSL) networks are beingdeployed by many service providers. The HFC architecture utilizesoptical fiber to transport data from the head-end to a curbside opticalnode in the neighborhood. The final distribution to the subscriber homesis performed by coaxial cable using a bus architecture. While this is arelatively low cost evolution of widely deployed CATV fiber-nodearchitecture, it suffers from very low throughput during peak hours dueto the large number of subscribers who share the bandwidth provided bythe optical node. DSL uses the same twisted pair as telephone lines andrequires a DSL modem at the customer premises and a digital subscriberline access multiplexer (DSLAM) in the central office. While the datarate (128 Kb/s -1.5 Mb/s) offered by DSL is significantly higher thanthat of an analog modem, it can hardly be considered as broadband as itcannot support full-service voice, data, and video. In addition, itsuffers the distance limitation as one central office can only coverdistances less than approximately 5.5 km using present technologies.

In the past several years, two technologies have begun to emerge asalternatives for optical access: (i) point-to-point Gigabit Ethernet,and (ii) passive optical network (PON). Point-to-point is the topologyEthernet has used successfully for a decade. It is a logical way todeploy optical fiber in the subscriber access network. However, withdedicated fiber running from the central office to each subscriber, thisarchitecture has very low fiber utilization (N fibers and 2Ntransceivers are needed for N subscribers). While the technology has itsmerits, such as very easy bandwidth provisioning and its excellence fornative LAN extension service, in many cases, it is viewed by many as aless attractive solution for small- to medium-sized businesses andresidences. An alternative to the point-to-point Ethernet is thecurb-switched Ethernet where a remote switch is deployed near theneighborhood. While this reduces the fiber consumption from N fibers to1 fiber, the number of transceivers increases by 2 to 2N+2. In addition,a curb-switched Ethernet requires active components and thereforeelectrical power in the field, an unfavorable situation due to, interalfa, high cost.

Passive optical networks (PONs) are low costFiber-to-the-Building/Curb/Home (FTTb, FTTc, FTTh, collectively referredto as FTTx) solutions. A PON is a point-to-multipoint optical networkthat allows service providers to minimize the need for fiber in theoutside portion of the network to interconnect buildings or homes. Thebasic principle of PON is to share the central optical line terminal(OLT) and the feeder-fiber by as many optical network units (ONUs) as ispractical. This resource sharing allows a significant reduction ofnetwork capital expense allocated to each subscriber and thereforeenables broadband fiber access in areas where achieving profitabilityhas been a formidable task for traditional point-to-point or ring-basedarchitectures.

A typical prior art PON model 100 is shown in FIG. 1, and consists offour elements: an optical line terminal (OLT) 102, a plurality ofoptical network units (ONUs) 104, an optical distribution network, alsoknow as the outside plant (OSP) 106, and an element management system(EMS) 108. The OLT 102 typically resides in the central office (CO),serving as the interface between the PON system and the serviceprovider's core networks 110. The ONUs 104 are located at either thecurb or the end-user location, serving as the interface between the PONsystem 100 and broadband service customers 112. The optical distributionnetwork 106 includes single-mode fiber optic cable, passive opticalsplitters/couplers, connectors and splices. The element managementsystem (EMS) 108 manages a plurality of PONs and there respective nodes.It offers network management functions in areas including faultdetection/isolation, configuration, accounting, performance, andsecurity. The OLT and ONUs are now described in more detail.

The OLT 102 in a typical asynchronous transfer mode (ATM) PON systemconsists of three parts: (i) the service port function; (ii) an ODNinterface; and (iii) a MUX for VP grooming.

The service port function serves as an interface to service nodes. Theservice port function inserts ATM cells into the upstream SONET/SDHpayload and extracts ATM cells from the SONET/SDH payload.

The optical distribution network interface performs optoelectronicconversion. It inserts ATM cells into the downstream PON payload andextracts ATM cells from the upstream PON payload.

Lastly, the MUX provides VP connections between the service portfunction and the ODN interface.

An ONU in an ATM PON system consists of an ODN interface, user port,transmission, customers and services, mux/demux functions, and powering.The ODN interface performs the optoelectronic conversion. The ODNinterface extracts ATM cells from the downstream payload and inserts ATMcells into the upstream PON payload. The MUX multiplexes serviceinterfaces to the ODN interface(s). The user port interfaces over UNI toa terminal, and inserts ATM cells into the upstream payload and extractsATM cells from the downstream payload. ONU powering is typicallyimplementation dependent.

The wavelength window of PON is typically in the 1.5 μm region fordownstream and 1.3 μm region for upstream to support a single fibersystem. Downstream traffic is transmitted from the central officetowards the optical star coupler where light signal is passively splitand distributed by a plurality of optical fibers to a plurality ofoptical network units (ONUs). The ONUs provide data, voice, and videoservices to the end subscriber(s) electronically. In the upstreamdirection, the respective signals from the ONUs are passively combinedby the optical star coupler. The combined optical signal is thendistributed to the central office through a single optical fiber. Someproposed PON schemes utilize wavelengths other than 1.5 μm/1.3 μm ormultiplex additional wavelengths to support an analog/digital videooverlay on the same fiber. Others use a second PON (video PON) toprovide video services. The video PON is typically provided on aparallel fiber that has the same physical layout as the first PON.

To date, PON-based optical access networks have primarily been designedto use asynchronous transfer mode (ATM) as its layer 2 protocol (ITUStd. G.983), and thus the term “APON.” ATM was chosen because it wasconsidered to be suitable for multiple protocols. In this scheme, bothdownstream and upstream data are formatted to fit into the fixed timeslot cell structure (e.g., 53 bytes) of ATM. In the downstreamdirection, data is broadcast at 1550 nm using Time Division Multiplexing(TDM) protocol for point-to-multipoint transmission. In the upstreamdirection, 1310 nm is used over which a Time Division Multiple Access(TDMA) protocol is applied providing the multipoint-to-point sharedmedium access.

More recently, however, Ethernet (IEEE Std. 802.3) has emerged as auniversally accepted standard, with several hundred millions of Ethernetports deployed worldwide. This large-scale deployment has steadilydriven the prices of standard Ethernet devices down. As of this writing,the deployment of Gigabit Ethernet is increasing and 10 Gigabit Ethernetproducts are becoming more and more available. In addition to itseconomic advantages, Ethernet is in many ways a logical candidate for anIP data optimized access network. An Ethernet PON (EPON) is a PON inwhich both downstream and upstream data are encapsulated in Ethernetframes. For the most part, an EPON is very similar to an APON (seeFIG. 1) in that the network topology is architecturally similar andadheres to many G.983 recommendations. In the downstream direction,Ethernet frames are broadcast at 1550 nm through the 1:N passive starcoupler and reach each ONU. Since broadcasting is one of the keycharacteristics of Ethernet, it makes logical sense to use Ethernetframes in the PON architecture. In the upstream direction, traffic ismanaged utilizing time-division multiple access (TDMA). In this scheme,transmission time slots are allocated to all of the ONUs. The time slotsare synchronized so that upstream data from the ONUs do not collide witheach other once the data are coupled onto the single common fiber. Forthe purposes of the present discussion, both APONs and EPONs arereferred to herein as PONs.

Although the art of transmitting data from central office to user andfrom user to central office is well developed based on either APON orEPON technology, certain problems still exist. In particular, in theupstream direction, due to the directional properties of the opticalstar coupler, data from any ONU 104 will only reach the OLT 102, and notother ONUs. Thus, simultaneously transmitted data from different ONUs104 use a time-sharing mechanism (i.e. TDMA) to avoid collision. Someother methods for avoiding upstream collisions include installing anEthernet hub at the star coupler (and thereby effectively defeating thepurpose of being passive), and using wave-division multiplexing (WDM) toseparate one ONU from another. This latter approach is quite costprohibitive. Due to the lack of their popularity, these latter twomethods (i.e., use of the Ethernet hub and WDM) are not discussedfurther herein.

While TDMA does provide the scheduling capability, it also imposes morecomplexity on hardware and protocol software. For example, both OLT andONU must be able to manage and process the transmitted and received datain terms of timeslots and frames as well as perform framesynchronization. Clearly, these requirements cannot be easily satisfiedby conventional Ethernet or non-Ethernet devices. In addition, in orderto avoid upstream frame collision in an APON (and presumably EPON aswell), the OLT 102 must perform an operation known as “ranging” in whichthe OLT measures the distance to each ONU 104, and then tells the ONU104 to insert the appropriate delay so that all equivalent OLT-ONTdistances are a predetermined value, e.g., 20 km. The ranging procedurecomplicates the protocol software significantly.

Another common problem associated with existing passive optical networksis the so-called “near-far” problem. This problem is caused by unequaldistances between the central office and various ONUs. The longer thedistance, the lower the power level received at the OLT 102. A number ofapproaches have been considered to overcome this problem. For example, aburst mode OLT receiver that is able to quickly adjust its zero-onethreshold at the beginning of each received time slot can be used todetect the incoming bit-stream correctly. Alternatively, a specialOLT-ONU signaling protocol can be developed that allows the ONUs toadjust their respective transmitter power based on OLT feedback suchthat power levels received by the OLT from all the ONUs are the same.While these methods do solve the near-far problem, they require highlysophisticated hardware and/or software, thereby increasing complexity aswell as cost of implementation.

It is noted that even in the case of EPON, as long as TDMA is used toresolve upstream frame collision, significant amount of hardware andsoftware components are required at both the OLT 102 and ONU 104. Thesecomponents are not the types used on enterprise Ethernet networks andtherefore may not benefit from the volume advantage of standard Ethernetdevices.

Additionally, it will be recognized that the operation of Ethernet islargely based on the contention-based media access protocol CSMA/CDalong with the back-off algorithm (10 Gigabit Ethernet not included).This MAC layer protocol has many desirable characteristics such assimplicity and being a well understood and proven technology. Therefore,it is highly desirable to operate the PON using Ethernet-like MACprotocols.

Based the foregoing, it is clear that a need exists for an improvedoptical networking architecture and methods that take advantage of thepopularity of widely accepted communications standards and protocols(such as Ethernet) and their high deployment scale. Ideally, sucharchitecture and methods would employ a simple medium access protocol(such as the aforementioned CSMA/CD), and allow the use of low-costnetwork interface devices based on these widely accepted standards andprotocols.

SUMMARY OF THE INVENTION

The present invention satisfies the aforementioned needs by an improvedapparatus and method for optical networking.

In a first aspect of the invention, an improved networking architectureis disclosed. This improved architecture generally comprises a firstnode being adapted for transmitting and receiving optical signals; afirst optical coupler in optical communication with the first node andadapted to split signals received by the first coupler; a second opticalcoupler in optical communication with the first node and adapted tosplit signals received by the second coupler, said second coupler alsobeing in optical communication with the first coupler; and a pluralityof second nodes in optical communication with both the first and secondcouplers, the plurality of nodes each being adapted for transmitting andreceiving optical signals; wherein the first coupler may be used tosplit signals received from one of the second nodes and communicatesignals to another of the nodes via the second coupler.

In one exemplary embodiment, the network apparatus comprises twointerconnected passive optical networks (PONs), with each networkemploying a 2×N (e.g., N=16 or 32) passive star coupler. An optical lineterminal (OLT) is connected to a first port of the first star couplerthrough a single feeder-fiber. Other ports of the first star coupler areconnected to a plurality of optical network units (ONUs) through anumber (N) of distribution fibers. Each of these ONUs is associated witha respective remote user or group of remote users. The connections inthe second passive optical network are essentially the same. In bothnetworks, the downstream traffic is broadcast by a transmitter at the COto the ONUs by the first and the second star couplers, respectively. Thefirst PON carries one or more first types of information (e.g., data,voice, and IP streaming video), while the second PON carries one or moresecond types of information (e.g., video). In this exemplary embodiment,the first PON is an Ethernet passive optical network (EPON) where allinformation (e.g., data, voice, and IP video) are encapsulated inEthernet frames. The second PON is a video passive optical network(VPON) where standard analog video content can be delivered along withdigital CATV over the RF spectrum. The first and second star couplersare interconnected by an optical fiber between corresponding portsthereof. In the upstream direction, an optical signal from an ONU (e.g.,with operating wavelength 1260-1360 nm) is coupled to a port of thefirst star coupler. This optical signal is then redirected to acorresponding port of the second star coupler. The redirected opticalsignal is broadcast to the ONUs equipped with optical receivers forlocal communications. The detection of optical signal by anothertransmitting ONU indicates that a collision has occurred. Hence, theapparatus not only provides integrated data, voice, and video servicesbut also allows all remote users to detect the upstream transmission byother users.

In the exemplary embodiment, an Ethernet CSMA/CD MAC can advantageouslybe implemented in the optical domain on a PON system. Additionally,local networking services can be provided to all the ONUs. Finally, bytaking full advantage of the highly symmetrical structure, the systemarchitecture can support feeder-fiber protection with minimum cost.

In a second aspect of the invention, an improved optical network unit isdisclosed. The optical network unit generally comprises: a first opticalnetwork interface adapted to transfer first signals between a first userdevice operating in a non-optical domain and an optical network; and asecond optical network interface adapted to transfer second signalsbetween a second user device operating in a non-optical domain and theoptical network. In one exemplary embodiment, the optical networkcomprises a PON, with the network unit designed to interface with localloop or user-end equipment. The unit converts signals into the opticaldomain and transmits them onto the network. The unit is also adapted toreceive optical video signals and convert them into the user-end domain(e.g., coaxial distribution) for use by the user(s). The unit alsoincludes a third interface for receiving “redirected” split opticalsignals from other units in the same network in order to facilitatearbitration and collision detection as previously described.

In a third aspect of the invention, an improved method for arbitratingamong a plurality of network units of a broadband optical network. Themethod generally comprises splitting first optical signals derived fromat least one of a plurality of network units using a first coupler toproduce at least second and third signals; splitting at least a portionof the second signals using a second coupler; and distributing at leasta portion of the split second signals to the plurality of network units.The split second signals allow the network units to implement an accessprotocol (such as the aforementioned CSMA/CD protocol), therebyarbitrating access to the network assets between themselves.

In a fourth aspect of the invention, an improved method for providingfeeder protection in an optical network is disclosed. The methodgenerally comprises: providing first and second optical couplers each inoptical communication with (i) a first node via respective first andsecond fibers, and (ii) each of a plurality of second nodes; utilizingat least the first fiber to transfer optical signals between the firstnode and the plurality of second nodes via the first coupler duringnon-fault conditions; and utilizing the second fiber to transfer opticalsignals between the first node and the plurality of second nodes via thesecond coupler during fault conditions on the first fiber.

In a fifth aspect of the invention, an improved method of controllingthe optical power applied by an optical source to an optical networkhaving a plurality of optical couplers is provided. The method generallycomprises: establishing an information resource (e.g., look-up table)having at least first information relating to the optical network;determining the path distance between the source and at least one of theoptical couplers; accessing the information resource using said pathdistance to determine a desired transmitter power level; and adjustingthe actual transmitter power level to the desired level if required.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objectives, and advantages of the invention will becomemore apparent from the detailed description set forth below when takenin conjunction with the drawings, wherein:

FIG. 1 is a functional block diagram of a typical passive opticalnetwork (e.g., APON or EPON) architecture according to the prior art.

FIG. 2 is a functional block diagram of one exemplary embodiment of theoptical network architecture according to the present invention.

FIG. 2a is a graphical representation illustrating the operation of a2×2 directional coupler according to the prior art.

FIG. 3 is block diagram illustrating a first exemplary embodiment of anOLT functional architecture according to the present invention.

FIG. 4 is block diagram illustrating a first exemplary embodiment of anONU functional architecture according to the present invention.

FIG. 5 is a functional block diagram of a first exemplary embodiment ofthe optical network architecture of the invention, illustratingdownstream traffic flow.

FIG. 6 is a functional block diagram of the optical network architectureof FIG. 5, illustrating upstream traffic flow.

FIG. 6a is a logical flow diagram illustrating an exemplary method fordetermining when an ONU can transmit onto the network.

FIG. 7a is a logical flow diagram illustrating one exemplary embodimentof the calibration methodology according to the present invention.

FIG. 7b is a logical flow diagram illustrating one exemplary embodimentof the power balancing methodology according to the present invention.

FIG. 8 is a functional block diagram of the optical network architectureof FIG. 5, illustrating the operation of local networking servicesaccording to the present invention.

FIG. 9 is a functional block diagram of the optical network architectureof FIG. 5, illustrating an exemplary arrangement for a 1:1 protectionswitch (downstream traffic only) according to the present invention.

FIG. 10 is a functional block diagram of the protection switcharrangement of FIG. 9, except with upstream traffic.

FIG. 11 is a functional block diagram of the protection switcharrangement of FIGS. 9 and 10, illustrating the operation of localnetworking in the case of a protection switch.

DETAILED DESCRIPTION

Reference is now made to the drawings wherein like numerals refer tolike parts throughout.

It is noted that while the following description is cast primarily interms of an exemplary Ethernet-based CSMA/CD system, other accesstechniques and protocols may be used in conjunction with or in place ofthe particular techniques described herein. Accordingly, the followingdiscussion of the Ethernet CSMA/CD approach is merely exemplary of thebroader concepts.

As used herein, the terms “splitting” and “split” refer generally to theprocess of dividing or reducing a signal into two or more constituentcomponents, and in no way is limited to an even division into two equalcomponents.

As used herein, the term “node” refers to any functional entityassociated with a network, such as for example an OLT or ONU, whetherphysically discrete or distributed across multiple locations.

Overview

The present invention includes methods and apparatus for providingenhanced optical networking service and performance which areparticularly advantageous in terms of low cost and use of existinginfrastructure and systems.

In the exemplary embodiment, the invention leverages the currentwidespread deployment and associated low cost of Ethernet-based systemsto provide the aforementioned benefits. Specifically, an EthernetCSMA/CD MAC is advantageously implemented in the optical domain on aconventional PON system. Additionally, local networking services can beprovided to all the network units (ONUs) on the network since each localreceiver can receive signals from all other users. A novel symmetriccoupler arrangement provides this functionality at low cost.

Finally, by taking full advantage of the aforementioned symmetricalstructure of the system architecture, the system can supportfeeder-fiber protection with minimum cost. Such protection strategygreatly improves the service and access network availability and yetmaintains the low-cost characteristic of the conventional PON.

Apparatus and Methods

Referring now to FIGS. 2-11, an exemplary embodiment of the presentinvention is described in detail. In this embodiment, a B-PONarchitecture is disclosed that enables a simplified (e.g., CSMA/CD) MACto be implemented in the optical domain such that no other complicatedMAC protocol is required. As a result, standard Ethernet devices canadvantageously be used with only minor optical and electronicadaptation. Another significant advantage of this approach is that theexisting PON architectures require very little change, and the number ofextra components required is minimum. The utilization of speciallyadapted optical couplers (e.g., two 2×N optical star couplers in theillustrated embodiment) are instrumental in providing these features.

FIG. 2 illustrates an exemplary network architecture according to theinvention. For clarity, the EMS, the service provider's core networks,and the end users are not shown. In this architecture 200, twoeffectively parallel optical couplers 202, 204 are provided between theOLT 300 (FIG. 3) and the plurality of ONUs 400 (FIG. 4) which form thenetwork. The respective optical couplers 202, 204 are dedicated tocorresponding or symmetrical portions of the network; i.e., the “data”portion 210 and the video portion 212. In the present embodiment, thedata portion 210 is based upon the Ethernet standard as previouslydiscussed, thereby comprising an Ethernet passive optical network(EPON). By providing both data and other (e.g., video) services in onePON, the architecture 200 of FIG. 2 is considered “converged” ormulti-functional.

FIG. 2a shows an exemplary embodiment of a 2×2 directional coupler 250that can be naturally generalized to a P×N star coupler. As is wellknown in the optical arts, a star coupler can be used to split andcombine signals in an optical network. It has the property that thepower from each input is equally divided among all the outputs. Theminimum theoretical insertion loss in dB of a P×N star coupler is givenby Eqn. 1:

L _(P×N)=10 log₁₀(max{P, N})   (Eqn. 1)

It is noted that optical star couplers are reciprocal devices, that is,they work exactly the same way if their inputs and outputs are reversed.

The optical line terminal (OLT) 300 in the optical network 200 istypically connected to the switched networks via standardizedinterfaces. At the distribution side, it presents optical accessesaccording to the agreed requirements, in terms of bit rate, powerbudget, etc. FIG. 3 shows an exemplary OLT architecture according to thepresent invention. In this configuration, the OLT 300 is connected to atelephony switch using an industry standard interface 302. In addition,the OLT is connected via a corresponding interface 304 to an IP routerfor data delivery. Finally, the OLT is connected via an interface 06 tovideo content suppliers to provide IP streaming video or RF videoservices. At the distribution side 308, the OLT 300 has two interfaces.The first interface 310 is for downstream and upstream Ethernet traffic,and the second interface 312 is for downstream video traffic only. Atthe first interface 310, Ethernet traffic is broadcast by the OLT 300through the first 2 ×N star coupler (not shown) and extracted by theappropriate destination ONU based on the MAC address. The upstreamtraffic from various ONUs is received by the OLT 300 at the sameinterface 310. Due to the broadcasting nature in the downstreamdirection and the contention-based media access control in the upstreamdirection (described in greater detail subsequently herein), theEthernet interface 310 at the OLT 300 operates in full-duplex mode (noCSMA/CD). Finally, video traffic is broadcast by the OLT through thesecond interface 312 and associated 2×N star coupler to the ONUs thatare connected to customer's analog and/or digital TV receiving devicessuch as set-top boxes, etc. In addition to the above, the OLT 300 isalso responsible for providing connections (or mapping) between theservice port function (such as an IP router interface) and the ODNinterface (such as the Ethernet interface to the PON). Techniques forproviding such mapping are well known to those of skill in the art, andaccordingly are not discussed in detail herein.

Referring now to FIG. 4, the exemplary ONU architecture of the inventionis described in detail. The ONU 400 provides the interface between thecustomer and the PON. The primary function of ONU is to receive trafficin optical format and convert it to the customer's desired format. AnONU is also responsible for transmitting upstream customer traffic tothe OLT 300. The ONU 400 has three optical distribution networkinterfaces 402, 404, 406. Through the first interface 402, the ONUconnects to an input port of the aforementioned first (star) coupler andis responsible for receiving downstream Ethernet traffic from the OLT aswell as transmitting upstream Ethernet traffic to the OLT 300 (EPON).For this interface 402, 1550 nm and 1310 nm wavelengths are used in thedownstream and upstream directions, respectively, although it will berecognized that other wavelengths may be used for either or bothdownstream and upstream directions if desired consistent with thepresent invention. Note that the upstream Ethernet traffic of theillustrated embodiment carries data and voice as well as other signalssuch as video-on-demand and channel change requests. The OLT 300 acts asthe coordinator between the Ethernet and video portions of the network(i.e., EPON and VPON, respectively, described subsequently herein).Through the second interface 404, the ONU 400 connects to an input portof the second star coupler and is responsible for receiving downstreamvideo traffic from the OLT 300 (VPON). A 1550 nm wavelength is used inthe present embodiment in the downstream direction for this interface.The third interface 406 of the ONU 400 provides the return path forupstream Ethernet traffic such that the CSMA/CD protocol can beimplemented. In addition, it enables local networking services among thevarious ONUs. The operating wavelength for this interface is the same asthat of the upstream traffic (1310 nm), albeit in the oppositedirection. The downstream video traffic and the redirected upstreamtraffic (now traveling downstream) both pass through the second starcoupler 204 and travel along the same distribution fiber. However, theyoperate at different wavelengths (1550 nm and 1310 nm, respectively). Inaddition to performing optoelectronic conversion, an ONU 400 also offersLayer 2 (or even Layer 3 ) switching capability, which allows customertraffic to be switched internally at the ONU 400. As in the case of ATMPON, an ONU may also include a number of other components (such as userport functions 420, 422) in addition to the ODN interface(s) 402, 404,406. The implementation details of such other components are well knownto those of ordinary skill, and accordingly not described furtherherein.

FIG. 5 shows that in the downstream direction, the optical signals arebroadcast by two different transmitters at the OLT 300 (e.g., located atthe CO) to all the ONUs 400 by the first and the second couplers 202,204 that are located within the first and second PONs 210, 212,respectively. As previously referenced herein, the difference betweenthese two network portions 210, 212 in the present embodiment is thatthe first PON 210 carries data, voice, and IP streaming video (operatingwavelength 1480-1580 nm), while the second PON 212 carries video(operating wavelength 1480-1580 nm). The first PON is an Ethernetpassive optical network (EPON) where all data, voice, and IP video areencapsulated in Ethernet frames. The second PON 212 is a video passiveoptical network (VPON) where, inter alia, standard analog video contentcan be delivered along with digital CATV over the RF spectrum. As far asdownstream traffic (from central office to user) is concerned, thearchitecture according to the embodiment of FIGS. 2-5 is effectivelyidentical to a co-located EPON/VPON combination in which both networkshave the same physical layout. It should be noted that the two feederfibers can also be routed through diverse paths so that they do not getfiber cut at the same time.

In the upstream direction, instead of using a time-division multipleaccess (TDMA) technique as in the prior art, the present embodiment usesa contention-based media access control protocol CSMA/CD to manage theoptical channel sharing among multiple ONUs 400. One of the four basicelements of Ethernet is the media access control (MAC) protocol; i.e.,the carrier sense multiple access/collision detection (CSMA/CD)protocol. This protocol is designed to provide fair access to the sharedchannel so that all users are provided an opportunity to access thenetwork and no user is locked out due to one or more other users“hogging” the channel. After every packet transmission, all users usethe CSMA/CD protocol to determine which user gets access to the Ethernetchannel next. It should be noted that CSMA/CD is only used inhalf-duplex mode of Ethernet operation. Half-duplex in the presentcontext simply means that only one user can send upstream data over theEthernet channel at any given time.

As shown in FIG. 6, all of the ONUs 400 are in the same collision domain606 and follow the same media access control rules. Being in the samecollision domain means they are all part of the signal timing domain inwhich if two or more devices transmit at the same time, a collision willoccur. The media access control rules refer to the CSMA/CD protocol.Note that the OLT 300 is not part of the collision domain in thisembodiment, and therefore does not use CSMA/CD protocol even though itis directly connected (not through an Ethernet switching hub) to therest of the network. The reason for not including OLT 300 in thecollision domain of this embodiment is that the ONUs 400 can performupstream bandwidth arbitration by utilizing CSMA/CD MAC entirely amongthemselves. Adding the OLT 300 to the collision domain has no additionalvalue in this context. In fact, since OLT 300 is the only source fordownstream traffic, there is no need for downstream bandwidtharbitration in the present embodiment. The OLT simply broadcasts to allthe ONUs whenever it wants to.

From the above discussion, it is clear that while the architectureaccording to the present invention is generally similar to an Ethernetnetwork, salient distinguishing features do exist. One such differencebetween an Ethernet architecture and that of the present embodiment isthat CSMA/CD MAC of the present embodiment is only used in the upstreamdirection. In addition, the exact implementations (such as carrierdetection) are different due to architectural differences between thetwo systems.

As in an Ethernet network, an ONU 400 must obey the following whentransmission of data is desired: i) the ONU 400 must know when it cantransmit; and ii) it must be able to detect and respond to a collision.

To determine when a given ONU 400 may transmit, the followingmethodology 650 is applied (referring to FIG. 6a ).

If there is no carrier (step 652) and the period of no carrier hascontinued for an amount of time that equals or exceeds the inter-framegap (IFG) per step 654, then the frame is transmitted immediately (step656). Note that for an ONU, no carrier means that there is no opticalsignal received by its local receiver.

If there is carrier (i.e. there is optical signal) per step 652, the ONU400 will continue to listen until the carrier becomes absent. As soon asthe carrier becomes absent (step 658), the ONU may begin the process oftransmitting a frame, which includes waiting for the inter-frame gap aspreviously described.

If a collision is detected during the transmission (step 660), the ONUwill continue to transmit a collision enforcement jam signal (step 662).Note that for an ONU, a collision is signaled by a difference betweenthe beginning of the received data and the data transmitted. Thedifference may be due to errors caused by colliding transmissions, orreception of an earlier transmission from another ONU, or a bit error onthe channel. After sending the jam signal, the ONU goes through thebackoff process (step 664). The operation and implementation of thebackoff algorithm are well known to those of ordinary skill, andaccordingly not described further herein.

Once an ONU 400 has transmitted a predetermined quantity of data (e.g.,512 bits of a frame for 100 Mbps assumed) without a collision (step666), the ONU is considered to have acquired the channel (step 668).After channel acquisition, the ONU simply continues the transmissionuntil the entire frame is transmitted (step 670).

It should be noted that although analogs of the above methodology havegenerally been used in Ethernet networks for many years, the presentinvention advantageously adapts this methodology to conventional passiveoptical networks, and specifically the architectures disclosed herein,for the first time.

In terms of required bandwidth, Fast Ethernet (100 Mbps) providesadequate bandwidth for contemplated applications occurring in theimmediate future. Gigabit Ethernet (1000 Mbps, or 1 Gbps) and even morecapable technologies developed subsequently hereto may be optionallyutilized in applications where additional bandwidth is required. Hence,the present invention can advantageously be readily adapted to (i) thebandwidth needs of the particular application; and (ii) accommodate newhigher data rate technologies, without altering the fundamentalarchitecture disclosed herein.

Ethernet standards provide configuration guidelines to ensure that theimportant Ethernet timing requirements are met, so that the MAC protocolwill function correctly. One of the requirements for Fast Ethernetconfiguration is that fiber segments must be less than or equal to 412meters in length. Similarly, for Gigabit Ethernet, segment lengths arelimited to 316 meters in order to meet the bit-timing budget of thesystem. These rules are implemented when configuring the ONU(s) 400, inaddition to other signal transmission considerations. That is, in theexemplary application of the invention employing Fast Ethernet, thedistance between one ONU 400 to another should be less than 412 m, andsimilarly in Gigabit applications, the ONU-ONU distance should bemaintained less than 316 m.

ITU Std. G.983.1 requires that the maximum range (i.e., distancesbetween each ONU and the OLT) of the PON is at least 20 km. While thisis certainly a relatively easy task for Fast Ethernet when operated infull-duplex mode (assuming single-mode fiber optic cable is employed),it is not necessarily as easy when Gigabit Ethernet is utilized, as thestandard specifies that a full-duplex 1000BASE-LX segment can reach asfar as 5000 meters (with single-mode fiber optic cable). Nevertheless,the IEEE 802.3ah Ethernet in the First Mile (EFM) Task Force is as ofthe present date defining a ≧10 km range with single single-mode fiber(SMF) as a standard for “Point to Point over Fiber”, and this standardcan be used as a reference for the present embodiment. In addition,vendors have developed “extended reach” versions of 1000BASE-LXsingle-mode interfaces that can send signals over distances of 70-100 kmor more. Therefore, in practice, a 20 km maximum range is notproblematic for the EPON 210 operating at Gigabit Ethernet speed. Itshould be noted that although the EPON portion 210 of the system 200according to the present invention does not operate in full-duplex modein a precise manner, at least the OLT 300 operates in full-duplex mode.In addition, since OLT 300 is not part of the collision domain, thedistances between each ONU 400 and the OLT do not have to be limited bythe Ethernet timing requirements. As a result, distances allowed byfull-duplex mode can be used between each ONU and the OLT.

An example downstream optical power budget calculation for the EPON 210of the illustrated embodiment is shown in Table 1 below.

TABLE 1 Mean loss Available Mean loss Quantity or sub-total power levelItems per unit (dB) length (dB) (dBm) Mean launched power −2.0 Splitterinsertion loss −14.5 1 −14.5 −16.5 ( 1/16) Splitter excess loss −1.0 1−1.0 −17.5 Splitter uniformity −3.0 1 −3.0 −20.5 Splitter polarization−0.6 1 −0.6 −21.1 dependent loss Fiber optic cable at 1550 nm −0.2 10−2.0 −23.1 (dB/km) Miscellaneous splices −2.0 1 −2.0 −25.1 Total opticalloss −23.1 Optical power level at the −25.1 receiver Minimum receiver−30.00 sensitivity (dBm)

An example upstream optical power budget calculation for the EPON 210 isshown in Table 2 below.

TABLE 2 Mean Quantity Mean loss Available loss per or sub-total powerlevel Items unit (dB) length (dB) (dBm) Mean launched power −2.0Splitter insertion loss −3.6 1 −3.6 −5.6 (½) Splitter excess loss −0.151 −0.15 −5.75 Splitter uniformity −0.8 1 −0.8 −6.55 Splitterpolarization −0.12 1 −0.12 −6.67 dependent loss Fiber optic cable at−0.35 10 −3.5 −10.17 1310 nm (dB/km) Miscellaneous splices −2.0 1 −2.0−12.17 Total optical loss −10.17 Optical power level at −12.17 thereceiver Minimum receiver −30.00 sensitivity (dBm) Note that in theexample of Table 2, a splitting ratio of 50/50 is used. A differentsplitting ratio (e.g. 25/75) can be used such that a different portionof optical power goes to the OLT.

An example optical power budget calculation for the return path of thesystem 200 is shown in Table 3 below.

TABLE 3 Mean loss Available Mean loss per Quantity or sub-total powerlevel Items unit (dB) length (dB) (dBm) Mean launched power −2.0 1^(st)splitter insertion loss −3.6 1 −3.6 −5.6 (½) 1^(st) splitter excess loss−0.15 1 −0.15 −5.75 1^(st) splitter uniformity −0.8 1 −0.8 −6.55 1^(st)splitter polarization −0.12 1 −0.12 −6.67 dependent loss 2^(nd) splitterinsertion loss −14.5 1 −14.5 −21.17 (1/16) 2^(nd) splitter excess loss−1.0 1 −1.0 −22.17 2^(nd) splitter uniformity −3.0 1 −3.0 −25.17 2^(nd)splitter polarization −0.6 1 −0.6 −25.77 dependent loss Fiber opticcable at 1310 nm −0.35 2 −0.7 −26.47 (dB/km) Miscellaneous splices −2.01 −2.0 −28.47 Total optical loss −26.47 Optical power level at −28.47the receiver Minimum receiver −30.00 sensitivity (dBm)FIGS. 7a-7b illustrate an exemplary methodology for power balancingaccording to the invention. As previously described, the near-farproblem is caused by unequal distances between the central office (CO)and various ONUs. The longer the distance, the lower the power levelreceived at the OLT 300. To overcome this problem, the power levels fromall the ONUs 400 shall be equalized to a pre-defined value at the inputports of the first star coupler. To achieve this goal, the ONU powerlevel differences due to varying distances as seen by the first coupler202 need to be compensated.

The power balancing methodology according to the present embodimentfirst performs a calibration process 700 (FIG. 7a ) to establish anaccessible resource (e.g., look-up table) per step 704 that contains atleast three types of information: (a) the ONU-to-coupler distance, (b)the ONU transmitter power level, and (c) the expected received powerfrom the return path. In the exemplary look-up table, this informationmay comprise respective columns or rows. The resource is designed toensure that for a given distance, there is a corresponding transmitterpower level that will lead to equalized power levels from all the ONUs400 at the input ports of the first (star) coupler. The resourceinformation is stored in an appropriate storage device (not shown) perstep 706 of FIG. 7 a.

When a new ONU 400 is powered on, as part of its initialization routine,it sends a message (i.e., “self-ranging” message) to itself per step 732of FIG. 7b through the signal return path and measures the round triptime which directly relates to the distance between the ONU 400 and thecoupler(s) 202, 204.

Once the distance between the ONU 400 and the star coupler 202, 204 isknown, the resource of FIG. 7a is accessed (step 734) to determine theappropriate transmitter power level, and the ONU transmitter power leveladjusted accordingly per step 736.

Next, per step 738, the power received from the return path is measuredand compared with the expected value. If these values match (within adetermined error band), the ONU transmitter power level is consideredbalanced, and the ONU enters the operational state (step 740). If thevalues do not match in step 738 due to reasons other than interferencesfrom other ONUs, the ONU transmitter power is re-adjusted in theappropriate direction (as determined by the sign of the difference ofthe comparison of step 738) for one or more additional times. If thecompared values still do not match after such re-adjustment(s), the ONUremains in the non-operational state and the failure is logged, reported, and/or other corrective action instituted.

FIG. 8 illustrates that the architecture according to the presentembodiment can be easily extended to provide “local” networkingservices. As used herein, local network services includes, for example,so-called local area networks (LAN), intranets, secure (“trusted”)networks, and virtual private networks (VPN). Such networking servicesmay be desirable, for example, in scenarios where the use of intranet isessential, such as in business offices or other enterprises, communitynetworking, and premises networking cases. The operation of the localnetwork 800 of the embodiment of FIG. 8 is similar to an Ethernet LAN.When one ONU 400 needs to communicate to another ONU, it follows thesame CSMA/CD rules as described previously herein. However, thedestination address of the receiving ONU is a local one. It should benoted that since the same transmitter is used for both upstream trafficand local traffic, the total upstream bandwidth can be shared betweenthe upstream traffic and the local traffic if local networking servicewere selected.

Referring now to FIGS. 9 and 10, the feeder-fiber protection aspects ofthe invention are described in detail. Specifically, as shown in FIGS. 9and 10, the symmetric system architecture 200 of the present inventioncan be utilized to provide protection switching in case of feeder-fiberfailure. As is well known in the fiber-optic communications arts, anunprotected feeder-fiber is a highly vulnerable part of a PON system. Afiber cut may cause loss of service to all users for significant amountof time (on the order of hours or even days). Therefore, it is highlydesirable that any PON architecture have some redundancy strategy inplace. The redundancy strategy according to the embodiment of FIG. 2 (asdetailed in FIGS. 9 and 10) is based on 1:1 (one-to-one) protection,although it will be recognized that other schemes of protection may beemployed. In 1:1 protection, two fibers are used between the source andthe destination. Traffic is transmitted over only one fiber at a time(normally referred to as the working fiber). If the working fiber iscut, the source and destination both switch over to the remaining (i.e.,protection) fiber. One of the main advantages of 1:1 over othertechniques such as “1+1” protection is that under normal operation, theprotection fiber can be used to transmit lower-priority traffic. Thislower-priority service will be discontinued if the working fiber is cut.

As shown in FIG. 9 and FIG. 10, the first feeder-fiber 902 connectingthe OLT 300 and the first coupler 202 is considered as the workingfiber, and the second feeder-fiber 904 connecting the OLT 300 and thesecond coupler 204 is considered as the protection fiber. Under normalconditions, the system operates as previously described with respect toFIGS. 2-8. However, when a fiber-cut 907 occurs on the working fiber902, the OLT 300 and the ONU(s) 400 will both detect the cut and switchto the protection fiber 904, and no automatic protection switching (APS)protocol is required. To enable the protection switch capability, asmall number of devices need to be added to the existing architecture200. Specifically, on the OLT side, a simple optical switch (not shown)can be utilized such that the signal can be switched from the workingfiber 902 to the protection fiber 904. On the ONU side, the receiveroriginally considered (for receiving local traffic only) now need to bereplaced by or modified with a transceiver that is the same type asthose being used for the ONU-OLT communication. In order to operateunder both normal and failure conditions, the two transceivers at theONU 400 must be capable of receiving optical signals properly in both1310 nm and 1550 nm wavelength ranges, respectively, which is readilyaccomplished using existing and well known optical technology.

Thus, a PON system that provides 1:1 feeder-fiber protection capabilityis created with very minimal modification and cost. In fact, if serviceand access network availability is the primary concern, thisfunctionality can be implemented in the PON at manufacture/installation.

It should also be noted that in addition to feeder fiber failure, thedistribution fiber(s) 920 could also fail. However, the impact ofdistribution fiber failure is much smaller, as such failures only affectthe connected ONU, and therefore only a limited number of users ascompared to the feeder fiber. There is a possibility that an ONU 400will perform a fiber switch due to a distribution fiber failure (asopposed to the feeder failure previously described). In this case,switching over to the protection fiber will not restore the traffic,since the OLT 300 is still operating on the working fiber. To handlethis situation, the ONU of the present embodiment is optionallyconfigured to simply switch back to the working fiber, and at the sametime, the OLT can declare that the affected ONU is out of service aftera predetermined time period of lost communication with the ONU.

As far as local networking services during a feeder fiber failure areconcerned, the ONUs 400 can still communicate with each other as if noprotection switch has occurred. However, the signal path 1100 under thiscircumstance (FIG. 11) is different than that previously shown in FIG.8.

It will be further recognized that while the foregoing discussion iscast in terms of a single PON 200 having an OLT 300, two couplers 202,204, and a plurality of ONUs 400, the present invention may be adaptedto larger or different architectures wherein, for example, multiple PONs200 are interconnected, or additional OLTs and/or couplers are utilized.

It should be noted that although in the aforementioned descriptionEthernet CSMA/CD MAC was used to perform access arbitration in theupstream direction, other techniques (such as TDMA) can be used as wellprovided certain pluggable circuit pack units at both the OLT and theONUs are replaced by appropriate units that support TDMAfunctionalities. In addition, a different set of software components maybe required at both the OLT and the ONUs, such software being readilyimplemented by those of ordinary skill given the present disclosure.

Furthermore, it will be recognized that the architecture of the presentinvention advantageously provides economies in terms of component usageand configuration. Specifically, it is noted that under the prior art,two separate or “stand alone” optical networks (i.e., a VPON and EPONconfigured as separate networks) will require an effectively equivalentnumber of components to the improved “connected” architecture of thepresent invention. Hence, the present invention affords theaforementioned enhanced functionality (i.e., collision detection,protection switch, and local networking services) without requiringadditional components of any significance.

Furthermore, the architecture of the present invention affords enhancedcost efficiency over the prior art, in that existing, lower cost“off-the-shelf” CSMA/CD and related components can be used as opposed tothe more costly and often more complex components used for upstreamarbitration in such prior art systems; e.g., a TDMA system requiringcomplex synchronization and related techniques.

It will be recognized that while certain aspects of the invention aredescribed in terms of a specific sequence of steps of a method, thesedescriptions are only illustrative of the broader methods of theinvention, and may be modified as required by the particularapplication. Certain steps may be rendered unnecessary or optional undercertain circumstances. Additionally, certain steps or functionality maybe added to the disclosed embodiments, or the order of performance oftwo or more steps permuted. All such variations are consideredencompassed within the invention disclosed and claimed herein.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device or process illustrated may be madeby those skilled in the art without departing from the invention. Theforegoing description is of the best mode presently contemplated ofcarrying out the invention. This description is in no way meant to belimiting, but rather should be taken as illustrative of the generalprinciples of the invention. The scope of the invention should bedetermined with reference to the claims.

1-38. (canceled)
 39. A computerized method of adjusting an optical powerof an optical network unit (ONU) of a passive optical network, thecomputerized method comprising: accessing data descriptive of at leastan expected received power level corresponding to an opticalcommunications path between the ONU and a first optical coupler of thepassive optical network; calculating a transmitter power level of theONU based at least on the accessed data descriptive of at least theexpected received power level; and adjusting a current transmitter powerlevel to the calculated transmitter power level.
 40. The computerizedmethod of claim 39, further comprising: transmitting a self-rangingmessage to the first optical coupler through the path; and receiving theself-ranging message from a second optical coupler; and determining around-trip distance of an optical communications path through the firstoptical coupler and the second optical coupler, the determining theround-trip distance comprising measuring: (i) a transmit timecorresponding to the transmitting of the self-ranging message to thefirst optical coupler and (ii) a receipt time of the self-rangingmessage from the second optical coupler; wherein the calculating of thetransmitter power level of the ONU is directly proportional to thedetermined round-trip distance.
 41. The computerized method of claim 39,further comprising determining whether a signal power received at theONU correlates with the expected received power level, the correlationbeing based at least on: (i) a difference of values between the signalpower received at the ONU and the expected received power level and (ii)a predetermined error range.
 42. The computerized method of claim 41,further comprising, when the difference of values falls outside of thepredetermined error range: readjusting, at least once, the currenttransmitter power level of the ONU; and comparing a difference of valuesbetween (i) a readjusted signal power received at the ONU and (ii) theexpected received power level.
 43. The computerized method of claim 42,further comprising, when the difference of values between (i) thereadjusted signal power received at the ONU and (ii) the expectedreceived power level falls outside of the predetermined error range,causing the ONU to enter a non-operational state, the non-operationalstate for implementing a corrective action.
 44. The computerized methodof claim 41, further comprising, when the difference of values fallsinside the predetermined error range, causing the ONU to enter anoperational state.
 45. The computerized method of claim 39, wherein theaccessing of the data further comprises accessing informationdescriptive of at least: (i) a distance between the ONU and the firstoptical coupler and (ii) the current transmitter power level of the ONU.46. An optical network unit (ONU) comprising: at least one opticalcommunications interface configured to perform data communication with(i) an optical access point of a passive optical network and (ii) anoptical coupler of the passive optical network; a transmitter comprisinga transmitter power associated therewith; processor apparatus in datacommunication with the at least one optical communications interface;and a non-transitory computer-readable storage apparatus in datacommunication with the processor apparatus and having at least onecomputer program, the least one computer program comprising a pluralityof instructions configured to, when executed by the processor apparatus,cause the ONU to: perform an initialization comprising: (i) transmissionof a message to the optical coupler, (ii) receipt of a message from theoptical coupler at the ONU, and (iii) determination of a distance to theoptical coupler based at least on measurements of a time taken for thetransmission to the optical coupler and a time taken for the return andreceipt by the ONU; access a data resource file comprising at least anexpected power level; and adjust the transmitter power level of the ONUbased at least on the expected power level and the determined distanceto the optical coupler.
 47. The optical network unit of claim 46,wherein the plurality of instructions are further configured to, whenexecuted by the processor apparatus, cause the ONU to compare a receivedpower level to the expected power level; wherein the ONU is configuredto, responsive to a value derived from the comparison between thereceived power level and the expected power level within a predeterminederror, enter an operational state; and wherein the ONU is furtherconfigured to, responsive to the value derived from the comparisonbetween the received power level and the expected power level beingoutside the predetermined error, readjust the transmitter power level.48. The optical network unit of claim 47, wherein the ONU is furtherconfigured to, when the readjustment of the transmitter power occurs:responsive to a value derived from a comparison between another receivedpower level and the expected power level within the predetermined error,enter the operational state; and responsive to the value derived fromthe comparison between other received power level and the expected powerlevel being outside the predetermined error, enter a non-operationalstate.
 49. The optical network unit of claim 48, wherein the entrance tothe non-operational state comprises an institution of a correctiveaction, the corrective action comprising one or more of (i) a log of thenon-operational state and/or (ii) a report of the non-operational state.50. The optical network unit of claim 46, wherein the ONU is configuredto perform a calibration prior to power-up of the ONU, the calibrationcomprising: determination of at least the expected power level, wherethe expected power level corresponds to an optical communications pathbetween the ONU and the optical coupler; and storage of informationrelated to at least the expected power level within the data resourcefile.
 51. The optical network unit of claim 46, wherein the at least oneoptical communications interface comprises at least: a first opticalcommunications interface configured to receive digital data from theoptical access point; and a second optical communications interfaceconfigured to transmit digital data from the ONU.
 52. The opticalnetwork unit of claim 51, wherein the passive optical network isconfigured to allow redirection of the digital data transmitted from theONU to a second ONU via a second optical coupler of the passive opticalnetwork.
 53. An optical network comprising: an optical line terminal(OLT) configured to exchange data with a switched network; a pluralityof optical couplers in data communication with the OLT; and an opticalnetwork unit (ONU), the ONU configured to: access data descriptive of atleast an expected received power level corresponding to an opticalcommunications pathway between the ONU and the plurality of opticalcouplers; determine a transmitter power level of the ONU based at leaston the data descriptive of at least the expected received power level;adjust a current transmitter power level of the ONU based on acomparison of the current transmitter power level with the determinedtransmitter power level; responsive to a comparison between the adjustedtransmitter power level and the expected power level within apredetermined error, enter an operational state; and responsive to thecomparison between the adjusted transmitter power level and the expectedpower level outside the predetermined error, readjust the transmitterpower level.
 54. The optical network of claim 53, wherein the pluralityof optical couplers comprises at least a P×N star coupler.
 55. Theoptical network of claim 53, wherein the ONU is configured to receiveone or more of: video traffic signals and/or data traffic signals. 56.The optical network of claim 55, wherein the OLT is configured totransmit the video traffic signals and/or the data traffic signals. 57.The optical network of claim 53, wherein the determined transmitterpower level is determined from a self-ranging process, the self-rangingprocess comprising a transmission of a message from the ONU to itselfvia the plurality of optical couplers.
 58. The optical network of claim53, wherein: the plurality of optical couplers comprises a first and asecond optical coupler; the ONU comprises a first ONU configured tocommunicate data with a second ONU; and the entrance to the operationalstate is configured to enable the first ONU and the second ONU tocommunicate data with each other via optical pathways, the opticalpathways being configured to provide optical signal communicationbetween the first and the second optical couplers.